1. Field of the Invention
The present invention relates to air data measurement systems and more particularly, to an air data measurement system having a pressure transducer and a circuit for linearizing the analog output of the pressure transducer with respect to the airspeed of an aircraft.
2. Description of the Related Art
In order to determine the airspeed of an aircraft, one needs to measure the impact pressure, Q.sub.c, of the airflow outside the aircraft. Impact pressure is defined as the difference between the total or Pitot pressure and the static pressure of the airflow. It varies exponentially with airspeed and is expressed as follows: EQU Q.sub.c =P.sub.S *(((1+(0.2*(A.sub.s /S).sup.2)).sup.3.5)-1)
where
P.sub.s =Pressure at Sea Level; PA1 A.sub.s =Airspeed; and PA1 S.sub.s =Speed of Sound.
The impact pressure is typically measured with a differential pressure transducer through a flexible diaphragm having a piezoresistive bridge attached thereon. The transducer is configured to generate a voltage signal corresponding to a deflection of the diaphragm. Due to the physical deflection characteristics of the flexible diaphragm, the pressure transducer cannot produce a voltage signal that is linearly proportional to pressures throughout its elastic deflection range. Thus, manufacturers specify a range in which the voltage signal is linearly proportional to a pressure range, that pressure range falling relatively within the elastic deflection range of the diaphragm. To measure accurately a complete range of pressures, users often employ a multitude of pressure transducers with different linear operating ranges. This solution, however, increases the cost and complexity of an air data measurement system.
There are prior art pressure transducers with extended linear ranges, i.e. capable of generating voltage signals that are linearly related to the deflection over an extended range of pressures, but they are expensive and thus have limited commercial applications. These pressure transducers are also undesirable as they have relatively large offset errors since the magnitude of the offset error is directly proportional to the pressure range of a transducer.
Furthermore, since the impact pressure Qc varies exponentially with airspeed, a differential pressure transducer would produce low voltage signals at low airspeed. These low voltage signals are susceptible to noise in the measurement system thereby rendering the low airspeed measurements inaccurate. Therefore, users often utilize costly and more sensitive pressure transducers for low airspeed range.
Accordingly, there is a need for an air data measurement system which utilizes a low-cost pressure transducer for an entire operating airspeed range of an aircraft and which provides highly accurate pressure measurements for low airspeed as well as for high airspeed.